Composite storage circuit and semiconductor device having the same composite storage circuit

ABSTRACT

An object of the present invention is to provide a compound storage circuit that includes a storage circuit including a volatile storage circuit and a nonvolatile storage circuit connected in parallel to each other and that is arranged to be capable of an instant-on function by storing information equal to storage information stored in the volatile storage circuit into the nonvolatile storage circuit, the compound storage circuit being capable of reducing power consumption, and a semiconductor device including the compound storage circuit. According to the present invention, in a compound storage circuit including a volatile storage circuit and a nonvolatile storage circuit connected in parallel to each other and a semiconductor device including the compound storage circuit, a determination circuit for comparing first storage information stored in the volatile storage circuit with second storage information that has already been stored in the nonvolatile storage circuit when storage information stored in the volatile storage circuit is written into the nonvolatile storage circuit is provided, and the first storage information is written into the nonvolatile storage circuit only when the first storage information is not equal to the second storage information.

TECHNICAL FIELD

The present invention relates to a compound storage circuit including avolatile storage circuit and a nonvolatile storage circuit that areconnected in parallel to each other, and to a semiconductor deviceincluding the compound storage circuit.

BACKGROUND ART

In a semiconductor device that is incorporated in an electronic computeror the like, such as a personal computer, and that performspredetermined processing, a volatile storage circuit has been providedaccording to need, so that the processing is performed while informationnecessary for the processing is sequentially stored into the volatilestorage circuit.

Such a volatile storage circuit stores information by supplying electricpower and has a characteristic of high writing and reading speed. Incontrast, the volatile storage circuit has a characteristic of losingstored information when power supply is interrupted due to a power-offoperation, a sudden power failure, or the like. Thus, when a powersource is turned on again after the interruption of the power supply,information stored before the interruption of the power supply does notexist in the volatile storage circuit. Therefore, information storedbefore the interruption of the power supply cannot be recovered.

Recently, an instant-on technology has been suggested. In the instant-ontechnology, a storage circuit includes a volatile storage circuit and anonvolatile storage circuit that are connected in parallel to eachother, and information equal to that stored in the volatile storagecircuit is stored into the nonvolatile storage circuit. Thus, when powersupply is interrupted due to a power-off operation, a sudden powerfailure, or the like, necessary information is stored into thenonvolatile storage circuit. Then, when the power supply resumes, astate before the interruption of the power supply can be immediatelyrecovered by using the information stored in the nonvolatile storagedevice.

However, in the above-mentioned storage circuit, even if informationthat has already been stored in the nonvolatile storage circuit is equalto information stored in the volatile storage circuit, writingprocessing of writing the information into the nonvolatile storagecircuit must be performed. Thus, unnecessary power is consumed.

In particular, due to the nonvolatility, nonvolatile storage circuitsrequire a lot of electric energy to write storage information. This is afactor preventing a reduction in power consumption.

DISCLOSURE OF INVENTION

In order to solve the above-mentioned problems, according to an aspectof the present invention, a compound storage circuit that includes avolatile storage circuit and a nonvolatile storage circuit connected inparallel to each other and that is arranged such that information equalto storage information stored in the volatile storage circuit is storedinto the nonvolatile storage circuit includes a determination circuitfor comparing first storage information stored in the volatile storagecircuit with second storage information that has already been stored inthe nonvolatile storage circuit when the storage information stored inthe volatile storage circuit is written into the nonvolatile storagecircuit. The first storage information is written into the nonvolatilestorage circuit only when the first storage information is not equal tothe second storage information.

In the compound storage circuit, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

In the compound storage circuit, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

In the compound storage circuit, when electric power supplied to thevolatile storage circuit is reduced, the storage information stored inthe volatile storage circuit is written into the nonvolatile storagecircuit. When power supply resumes after the electric power supplied isreduced, the storage information stored in the nonvolatile storagecircuit is returned into the volatile storage circuit.

In the compound storage circuit, each of the volatile storage circuitand the nonvolatile storage circuit includes power source supply meansthat operates when the electric power supplied is reduced.

In the compound storage circuit, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

In the compound storage circuit, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

In the compound storage circuit, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

According to another aspect of the present invention, a semiconductordevice including a compound storage circuit that includes a volatilestorage circuit and a nonvolatile storage circuit connected in parallelto each other and that is arranged such that information equal tostorage information stored in the volatile storage circuit is storedinto the nonvolatile storage circuit includes a determination circuitfor comparing first storage information stored in the volatile storagecircuit with second storage information that has already been stored inthe nonvolatile storage circuit when the storage information stored inthe volatile storage circuit is written into the nonvolatile storagecircuit. The first storage information is written into the nonvolatilestorage circuit only when the first storage information is not equal tothe second storage information.

In the semiconductor device, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

In the semiconductor device, the nonvolatile storage circuit includes amagnetic tunnel junction element as storage means.

In the semiconductor device, when electric power supplied to thevolatile storage circuit is reduced, the storage information stored inthe volatile storage circuit is written into the nonvolatile storagecircuit. When power supply resumes after the electric power supplied isreduced, the storage information stored in the nonvolatile storagecircuit is returned into the volatile storage circuit.

In the semiconductor device, each of the volatile storage circuit andthe nonvolatile storage circuit includes power source supply means thatoperates when the electric power supplied is reduced.

In the semiconductor device, the nonvolatile storage circuit includes amagnetic tunnel junction element as storage means.

In the semiconductor device, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

In the semiconductor device, a magnetic tunnel junction element is usedas storage means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram showing a compound storage circuit accordingto the present invention.

FIG. 2 is a timing chart for explaining a process performed by thecompound storage circuit shown by the circuit diagram in FIG. 1.

FIG. 3 is a timing chart for explaining a process performed by thecompound storage circuit shown by the circuit diagram in FIG. 1.

BEST MODE FOR CARRYING OUT THE INVENTION

According to a compound storage circuit and a semiconductor deviceincluding the compound storage circuit of the present invention, thecompound storage circuit includes a volatile storage circuit and anonvolatile storage circuit that are connected in parallel to eachother, and information equal to that stored in the volatile storagecircuit is stored into the nonvolatile storage circuit.

The compound storage circuit also includes a determination circuit forcomparing first storage information stored in the volatile storagecircuit with second storage information that has already been stored inthe nonvolatile storage circuit and for determining whether or not thefirst storage information is equal to the second storage informationwhen the information stored in the volatile storage circuit is writteninto the nonvolatile storage circuit.

When the determination circuit determines that the first storageinformation is not equal to the second storage information, the firststorage information is written into the nonvolatile storage circuit.When the determination circuit determines that the first storageinformation is equal to the second storage information, the firststorage information is not written into the nonvolatile storage circuit.

Thus, since writing into the nonvolatile storage circuit is notperformed when the first storage information is equal to the secondstorage information, the number of writing times into the nonvolatilestorage circuit can be reduced. Therefore, power consumption can bereduced.

In particular, in a case where storage information stored in thevolatile storage circuit is written into the nonvolatile storage circuitwhen electric power supplied to the volatile storage circuit is reduced,the storage information stored in the volatile storage circuit can bewritten into the nonvolatile storage circuit only when the storageinformation stored in the volatile storage circuit is likely to be lostdue to the reduction in the electric power supplied.

Accordingly, in the compound storage circuit, an occurrence of writingprocessing of writing information to be overwritten by the subsequentwriting processing without being read from the nonvolatile storagecircuit is prevented. Thus, the power consumption can be reduced.

Information to be written into the nonvolatile storage circuit isinformation used when power supply resumes after the power supply isreduced. The information written into the nonvolatile storage circuit iswritten into the volatile storage circuit when the power supply resumes.Thus, necessary information can be used from the volatile storagecircuit, which has a high reading speed, and a quick instant-on functioncan be realized.

A semiconductor device including the above-described compound storagecircuit formed on a semiconductor substrate is capable of storing anoperation state when power supply is interrupted by using the compoundstorage circuit. In addition, when the power supply resumes, a stateimmediately before the interruption of the power supply can be quicklyrecovered by using information stored in the compound storage circuit.

Thus, if the semiconductor device constitutes an electronic apparatus oran electric apparatus, the instant-on function can be readily realized.

In this case, the volatile storage circuit and the nonvolatile storagecircuit are not necessarily formed on the same semiconductor substrate.The volatile storage circuit and the nonvolatile storage circuit may beformed on respective semiconductor substrates, so that the volatilestorage circuit and the nonvolatile storage circuit can be connected inparallel to each other using desired wiring.

Embodiments of the present invention will be described with reference tothe drawings. In particular, explanations will be given in accordancewith the following order:

-   -   1) Explanation for Compound Storage Circuit    -   2) Explanation for Process Performed by Compound Storage Circuit        when Power Supply is Interrupted    -   3) Explanation for Process Performed by Compound Storage Circuit        when Power Supply Resumes

In the following description, a magnetic storage circuit including amagnetic memory is used in a nonvolatile storage circuit. However, amagnetic storage circuit is not necessarily used. The nonvolatilestorage circuit may include an EEPROM, a flash memory, or aferroelectric memory.

1) Explanation for Structure of Compound Storage Circuit

FIG. 1 is a circuit diagram showing a compound storage circuit 1according to an embodiment of the present invention. The compoundstorage circuit 1 includes a volatile storage circuit 2 and anonvolatile storage circuit 3 that are connected in parallel to eachother using a first connection line 4 and a second connection line 5.

In particular, the nonvolatile storage circuit 3 is connected to thevolatile storage circuit 2 via a determination circuit 6. As describedbelow, when first storage information stored in the volatile storagecircuit 2 is written into the nonvolatile storage circuit 3, thedetermination circuit 6 compares the first storage information withsecond storage information that has already been stored in thenonvolatile storage circuit 3, and the first storage information iswritten into the nonvolatile storage circuit 3 only when the firststorage information is not equal to the second storage information.

In this embodiment, the volatile storage circuit 2, the nonvolatilestorage circuit 3, and the determination circuit 6 are provided on asemiconductor substrate, and the volatile storage circuit 2 is alatch-type storage circuit arranged in a system LSI chip.

The volatile storage circuit 2 includes a power source 7 for informationholding constituted by a capacitor and a first switching transistor 8for controlling the operation of the power source 7 for informationholding. A power switch signal input line 9 is connected to the gate ofthe first switching transistor 8. The first switching transistor 8 iscontrolled in accordance with a power switch signal input from the powerswitch signal input line 9, so that the operation of the power source 7for information holding is controlled.

A first conducting wire 10 and a second conducting wire 11 connected toother storage circuits and elements are connected to the volatilestorage circuit 2. A first circuit selection switch 13 and a secondcircuit selection switch 14 each connected to a power source separationsignal input line 12 are disposed in the first connecting wire 10 andthe second conducting wire 11, respectively. In accordance with an inputof a power source separation signal from the power source separationsignal input line 12, the first circuit selection switch 13 and thesecond circuit selection switch 14 are turned on and off.

The first connection line 4 whose one end is connected to the firstconducting wire 10 is connected to the first conducting wire 10 betweenthe volatile storage circuit 2 and the first circuit selection switch13. Also, the second connection line 5 whose one end is connected to thesecond conducting wire 11 is connected to the second conducting wire 11between the volatile storage circuit 2 and the second circuit selectionswitch 14.

Also, a third circuit selection switch 16 connected to a read signalinput line 15 is disposed in the first connection line 4. In accordancewith an input of a read signal from the read signal input line 15, thethird circuit selection switch 16 is turned on and off.

A magnetic storage circuit is used in the nonvolatile storage circuit 3,as described above. Information of “0” or “1” is stored using magnetictunnel junction elements M. Since the latch-type storage circuit as thevolatile storage circuit 2 stores 2-bit information, two magnetic tunneljunction elements M are provided so that the nonvolatile storage circuit3 can store 2-bit information.

The magnetic tunnel junction elements M are connected to a first readline 17 and a second read line 18. The magnetic tunnel junction elementsM are connected to an information reading circuit 3 a via the first readline 17 and the second read line 18. Accordingly, information from themagnetic tunnel junction elements M can be read.

A first read control switching transistor 19 connected to the readsignal input line 15 is disposed in the first read line 17, and a secondread control switching transistor 20 connected to the read signal inputline 15 is disposed in the second read line 18. Information can be readfrom the magnetic tunnel junction elements M using the informationreading circuit 3 a, as described below, by inputting a read signal tothe read signal input line 15.

The information reading circuit 3 a includes a power source 21 forreading constituted by a capacitor. The information reading circuit 3 aalso includes a second switching transistor 22 for controlling theoperation of the power source 21 for reading. The gate of the secondswitching transistor 22 is connected to the power switch signal inputline 9. The second switching transistor 22 is controlled in accordancewith a power switch signal input from the power switch signal input line9, so that the operation of the power source 21 for reading iscontrolled.

In this embodiment, the nonvolatile storage circuit 3 also includes anequalizing switching transistor 24 whose base is connected to anequalizing signal input line 23. One end of the equalizing switchingtransistor 24 is connected to the first read line 17, and the other endof the equalizing switching transistor 24 is connected to the secondread line 18.

The determination circuit 6 includes a comparison determination section6 a and a writing driver circuit 6 b. The comparison determinationsection 6 a includes a plurality of AND gate circuits. The writingdriver circuit 6 b performs writing into the nonvolatile storage circuit3 in accordance with a result determined by the comparison determinationsection 6 a.

The comparison determination section 6 a includes a first AND gatecircuit 26 and a second AND gate circuit 27. The first AND gate circuit26 is connected to a write signal input line 25 to receive a writesignal, and receives a signal of second storage information stored inthe volatile storage circuit 3. The second AND gate circuit 27 receivesan output signal output from the first AND gate circuit 26, and receivesa signal of first storage information stored in the volatile storagecircuit 2 via the second connection line 5. The comparison determinationsection 6 a also includes a third AND gate circuit 28 and a fourth ANDgate circuit 29. The third AND gate circuit 28 is connected to the writesignal input line 25 to receive a write signal, and receives a signal ofsecond storage information stored in the volatile storage circuit 3. Thefourth AND gate circuit 29 receives an output signal output from thethird AND gate circuit 28, and receives a signal of first storageinformation stored in the volatile storage circuit 2 via the firstconnection line 4.

Thus, when the comparison determination section 6 a determines thatfirst storage information stored in the volatile storage circuit 2 isequal to second storage information stored in the nonvolatile storagecircuit 3, the writing driver circuit 6 b does not operate. When firststorage information stored in the volatile storage circuit 2 is notequal to second storage information stored in the nonvolatile storagecircuit 3, the writing driver circuit 6 b operates, and a predeterminedcurrent for writing is caused to flow in a write line 30 that isconnected to the magnetic tunnel junction elements M, so that the firststorage information is written into the magnetic tunnel junctionelements M.

The writing driver circuit 6 b includes a power source 31 for writingconstituted by a capacitor. When power supply is interrupted, apredetermined current for writing is caused to flow in the write line30, which is connected to the magnetic tunnel junction elements M, for apredetermined period, so that writing the first storage information intothe magnetic tunnel junction elements M can be ensured.

Normally, a system LSI chip includes many storage circuits 1, and firststorage information stored in the volatile storage circuit 2 is equal tosecond storage information stored in the nonvolatile storage circuit 3with 50% probability, in terms of the whole system LSI chip. Thus, sinceproviding the determination circuit 6 reduces the number of writingtimes into the nonvolatile storage circuit 3 by approximately half interms of the whole system LSI chip, power consumption can be reduced.

2) Explanation for Process Performed by Compound Storage Circuit whenPower Supply is Interrupted

A process performed by the compound storage circuit 1 when power supplyis interrupted will be described with reference to a timing chart shownin FIG. 2. Not only turning off of a main power source due to a shutdownoperation but also a power failure or unexpected trouble causesinterruption of power supply. In the following explanations, a casewhere a main power source is turned off due to a shutdown operation,which is a general power supply interruption state, will be described.The same process is performed when the power supply is interrupted,irrespective of the reason of the interruption of the power supply.

FIG. 2(a) is a timing cart of electric power in accordance with turningoff of a main power source of a system LSI chip including the compoundstorage circuit 1. When the amount of electric power supplied to thesystem LSI chip is less than or equal to a predetermined value due tothe turning off of the main power source, a power-off signal generationcircuit (not shown) of the system LSI chip operates. The power-offsignal generation circuit generates a power-off signal, as shown in FIG.2(b).

A power source separation signal generation circuit (not shown) operatesin accordance with the power-off signal. The power source separationsignal generation circuit generates a power source separation signal, asshown in FIG. 2(c).

The power source separation signal is input to the first circuitselection switch 13 and the second circuit selection switch 14 via thepower source separation signal input line 12. The first circuitselection switch 13 and the second circuit selection switch 14 cut offthe first conducting wire 10 and the second conducting wire 11,respectively.

Cutting off the first conducting wire 10 and the second conducting wire11 causes the volatile storage circuit 2 to be independent from otherstorage circuits and elements connected via the first conducting wire 10and the second conducting wire 11. Thus, information is prevented frombeing input to the volatile storage circuit 2, and storage informationstored in the volatile storage circuit 2 is inhibited from being changedafter the power supply is turned off.

In other words, if the first conducting wire 10 and the secondconducting wire 11 are connected to the other storage circuits andelements, an electric charge is dissipated from a drain side of atransistor constituting the volatile storage circuit 2 due to areduction in the electric power supplied to the volatile storage circuit2. Thus, storage information may be autonomously changed.

Although a transfer gate is used for the first circuit selection switch13 and the second circuit selection switch 14, any arrangement similarto a transfer gate capable of preventing electric charge dissipation ofa transistor constituting the volatile storage circuit 2 is possible.

A power switch signal generation circuit (not shown) operates inaccordance with the power-off signal. The power switch signal generationcircuit generates a power switch signal, as shown in FIG. 2(d).

The power switch signal is input to the first switching transistor 8 viathe power switch signal input line 9 to switch a power source for thevolatile storage circuit 2 to the power source 7 for informationholding. Also, the power switch signal is input to the second switchingtransistor 22 via the power switch signal input line 9 to switch a powersource for the information reading circuit 3 a to the power source 21for reading.

By operating the power source 7 for information holding and the powersource 21 for reading, the volatile storage circuit 2 is capable ofstoring first storage information for a predetermined period and thenonvolatile storage circuit 3 is capable of reading second informationinto the comparison determination section 6 a of the determinationcircuit 6, even if power supply is interrupted due to turning off of themain power source.

At the same time as generation of the power source separation signal andthe power switch signal in accordance with the power-off signal, a writesignal generation circuit (not shown) operates in accordance with thepower-off signal. The write signal generation circuit generates a writesignal, as shown in FIG. 2(e).

The write signal is input to the first AND gate circuit 26 and the thirdAND gate circuit 28 of the comparison determination section 6 a via thewrite signal input line 25. The comparison determination section 6 acompares first storage information read from the volatile storagecircuit 2 with second storage information read from the nonvolatilestorage circuit 3.

When the first storage information is equal to the second storageinformation, there is no need to write the first storage informationinto the nonvolatile storage circuit 3. Thus, the process terminateswithout operating the writing driver circuit 6 b.

When the first storage information is not equal to the second storageinformation, the writing driver circuit 6 b operates to write the firststorage information into the nonvolatile storage circuit 3. Apredetermined current for writing is caused to flow in the write line30, and the first storage information stored in the volatile storagecircuit 2 is written into the nonvolatile storage circuit 3, as shown inFIG. 2(f).

By providing the power source 31 for writing in the writing drivercircuit 6 b, the writing driver circuit 6 b can be operated for apredetermined period and the first storage information can be writteninto the nonvolatile storage circuit 3, even if the power supply isinterrupted due to turning off of the main power source.

If magnetic tunnel junction elements M are used for a magnetic storagecircuit in the nonvolatile storage circuit 3, only a short period, suchas about dozens of nanoseconds, is required for writing. Thus, thecapacity of the power source 7 for information holding, the power source21 for reading, and the power source 31 for writing, functioning aselectric power storing means, can be reduced.

The power source 7 for information holding, the power source 21 forreading, and the power source 31 for writing need the capacity forperforming writing processing into the nonvolatile storage circuit 3. Asshown in FIGS. 2(f) and 2(g), until writing of first storage informationinto the nonvolatile storage circuit 3 is completed, the volatilestorage circuit 2 stores the first storage information.

The above-described process is performed by the compound storage circuit1 when the main power source is turned off. Accordingly, by writingfirst storage information stored in the volatile storage circuit 2 intothe nonvolatile storage circuit 3 using a power-off signal, as atrigger, generated due to a reduction in the power supply, onlyinformation necessary when the power supply resumes after theinterruption of the power supply can be stored into the nonvolatilestorage circuit 3. Thus, the number of writing times into thenonvolatile storage circuit 3 can be reduced. Therefore, powerconsumption can be reduced.

3) Explanation for Process Performed by Compound Storage Circuit whenPower Supply Resumes

A process performed by the compound storage circuit 1 when power supplyresumes after the power supply is interrupted due to turning off of themain power source will be described with reference to a timing chartshown in FIG. 3.

FIG. 3(a) is a timing chart of electric power supplied from the powersource to the system LSI chip including the compound storage circuit 1in accordance with resumption of power supply. When the amount ofelectric power supplied to the system LSI chip reaches a predeterminedvalue, a power-on signal generation circuit (not shown) of the systemLSI chip operates. The power-on signal generation circuit generates apower-on signal, as shown in FIG. 3(b).

The power source separation signal generation circuit (not shown)operates in accordance with the power-on signal. The power sourceseparation signal generation circuit generates a power source separationsignal, as shown in FIG. 3(c).

The power source separation signal is input to the first circuitselection switch 13 and the second circuit selection switch 14 via thepower source separation signal input line 12. The first circuitselection switch 13 and the second circuit selection switch 14 cut offthe first conducting wire 10 and the second conducting wire 11,respectively.

Cutting off the first conducting wire 10 and the second conducting wire11 causes the volatile storage circuit 2 to be independent from otherstorage circuits and elements connected via the first conducting wire 10and the second conducting wire 11. Thus, second storage informationstored in the nonvolatile storage circuit 3 can be written into thevolatile storage circuit 2 without being affected by the other storagecircuits and elements.

As shown in FIG. 3(d), a read signal generation circuit (not shown)generates a read signal in synchronization with start of power supply tothe system LSI chip. The read signal is input to the first read controlswitching transistor 19 and the second read control switching transistor20, as well as the third circuit selection switch 16, via the readsignal input line 15.

Then, in accordance with the power-on signal, an equalizing signalgeneration circuit (not shown) generates an equalizing signal, as shownin FIG. 3(e), and inputs the equalizing signal to the equalizing signalinput line 23.

In accordance with an input of the equalizing signal to the equalizingsignal input line 23, equalization is performed for the volatile storagecircuit 2, as shown in FIG. 3(f). Here, in the nonvolatile storagecircuit 3, second storage information is read from the magnetic tunneljunction elements M into the information reading circuit 3 a inaccordance with the read signal.

By disabling the equalizing signal and inputting the read signal, thesecond storage information stored in the nonvolatile storage circuit 3is read into the volatile storage circuit 2, as shown in FIG. 3(f).

By disabling the power source separation signal after the second storageinformation is read into the volatile storage circuit 2, the firstconducting wire 10 and the second conducting wire 11 that are cut off bythe first circuit selection switch 13 and the second circuit selectionswitch 14, respectively, are connected. Thus, the second storageinformation read into the volatile storage circuit 2 and the volatilestorage circuit 2 itself become usable.

The above-described process is performed by the compound storage circuit1 when power supply resumes. Accordingly, by returning storageinformation stored in the nonvolatile storage circuit into the volatilestorage circuit when power supply resumes, necessary information can beused from the volatile storage circuit, which has a high reading speed.Thus, a quick instant-on function can be realized.

INDUSTRIAL APPLICABILITY

(1) According to an aspect of the present invention, a compound storagecircuit that includes a volatile storage circuit and a nonvolatilestorage circuit connected in parallel to each other and that is arrangedsuch that information equal to storage information stored in thevolatile storage circuit is stored into the nonvolatile storage circuitincludes a determination circuit for comparing first storage informationstored in the volatile storage circuit with second storage informationthat has already been stored in the nonvolatile storage circuit when thestorage information stored in the volatile storage circuit is writteninto the nonvolatile storage circuit. The first storage information iswritten into the nonvolatile storage circuit only when the first storageinformation is not equal to the second storage information.

Thus, since the number of writing times into the nonvolatile storagecircuit can be reduced, power consumption can be reduced.

(2) In the compound storage circuit, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

Thus, the determination circuit can perform determination processingquickly. When the first storage information is not equal to the secondstorage information, writing the first storage information into thenonvolatile storage circuit can be ensured.

(3) In the compound storage circuit, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

Thus, when the first storage information is-written into the nonvolatilestorage circuit, writing can be performed in a relatively short time.

(4) In the compound storage circuit, when electric power supplied to thevolatile storage circuit is reduced, the storage information stored inthe volatile storage circuit is written into the nonvolatile storagecircuit. When power supply resumes after the electric power supplied isreduced, the storage information stored in the nonvolatile storagecircuit is returned into the volatile storage circuit.

Thus, only when storage information stored in the volatile storagecircuit is likely to be lost due to a reduction in power supply, writingthe storage information stored in the volatile storage circuit into thenonvolatile storage circuit can be ensured. This reduces the number ofwriting times into the nonvolatile storage circuit, thus reducing powerconsumption. In addition, information to be written into the nonvolatilestorage circuit is information necessary when power supply resumes afterthe reduction in the power supply. In addition, by returning the storageinformation stored in the nonvolatile storage circuit into the volatilestorage circuit when the power supply resumes, necessary information canbe used from the volatile storage circuit, which has a high readingspeed. Thus, a quick instant-on function can be realized.

(5) In the compound storage circuit, each of the volatile storagecircuit and the nonvolatile storage circuit includes power source supplymeans that operates when the electric power supplied is reduced.

Thus, even if the power supply is reduced, the power source supply meanscan operate the volatile storage circuit and the nonvolatile storagecircuit for a predetermined period. Accordingly, storing necessaryinformation into the nonvolatile storage circuit can be ensured.

(6) In the compound storage circuit, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

Thus, when the first storage information is written into the nonvolatilestorage circuit, writing can be performed in a relatively short time.

(7) In the compound storage circuit, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

Thus, the determination circuit can perform determination processingquickly. When the first storage information is not equal to the secondstorage information, writing the first storage information into thenonvolatile storage circuit can be ensured.

(8) In the compound storage circuit, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

Thus, when the first storage information is written into the nonvolatilestorage circuit, writing can be performed in a relatively short time.

(9) According to another aspect of the present invention, asemiconductor device including a compound storage circuit that includesa volatile storage circuit and a nonvolatile storage circuit connectedin parallel to each other and that is arranged such that informationequal to storage information stored in the volatile storage circuit isstored into the nonvolatile storage circuit includes a determinationcircuit for comparing first storage information stored in the volatilestorage circuit with second storage information that has already beenstored in the nonvolatile storage circuit when the storage informationstored in the volatile storage circuit is written into the nonvolatilestorage circuit. The first storage information is written into thenonvolatile storage circuit only when the first storage information isnot equal to the second storage information.

Thus, since the number of writing times into the nonvolatile storagecircuit can be reduced, power consumption in the semiconductor devicecan be reduced.

(10) In the semiconductor device, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

Thus, the determination circuit can perform determination processingquickly. When the first storage information is not equal to the secondstorage information, writing the first storage information into thenonvolatile storage circuit can be ensured.

(11) In the semiconductor device, the nonvolatile storage circuitincludes a magnetic tunnel junction element as storage means.

Thus, when the first storage information is written into the nonvolatilestorage circuit, writing can be performed in a relatively short time.

(12) In the semiconductor device, when electric power supplied to thevolatile storage circuit is reduced, the storage information stored inthe volatile storage circuit is written into the nonvolatile storagecircuit. When power supply resumes after the electric power supplied isreduced, the storage information stored in the nonvolatile storagecircuit is returned into the volatile storage circuit.

Thus, in the semiconductor device, only when storage information storedin the volatile storage circuit is likely to be lost due to a reductionin power supply, writing the storage information stored in the volatilestorage circuit into the nonvolatile storage circuit can be ensured.This reduces the number of writing times into the nonvolatile storagecircuit, thus reducing power consumption in the semiconductor device. Inaddition, information to be written into the nonvolatile storage circuitis information necessary when the power supply resumes after thereduction in the power supply. In addition, by returning the storageinformation stored in the nonvolatile storage circuit into the volatilestorage circuit when power supply resumes, necessary information can beused from the volatile storage circuit, which has a high reading speed.Thus, a quick instant-on function can be realized.

(13) In the semiconductor device, each of the volatile storage circuitand the nonvolatile storage circuit includes power source supply meansthat operates when the electric power supplied is reduced.

Thus, even if the power supply is reduced, the power source supply meanscan operate the volatile storage circuit and the nonvolatile storagecircuit for a predetermined period. Accordingly, storing necessaryinformation into the nonvolatile storage circuit can be ensured.

(14) In the semiconductor device, a magnetic tunnel junction element isused as storage means.

Thus, when the first storage information is written into the nonvolatilestorage circuit, writing can be performed in a relatively short time.

(15) In the semiconductor device, the determination circuit includescomparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.

Thus, the determination circuit can perform determination processingquickly. When the first storage information is not equal to the secondstorage information, writing the first storage information into thenonvolatile storage circuit can be ensured.

(16) In the semiconductor device, a magnetic tunnel junction element isused as storage means.

Thus, when the first storage information is written into the nonvolatilestorage circuit, writing can be performed in a relatively short time.

1. A compound storage circuit that includes a volatile storage circuitand a nonvolatile storage circuit connected in parallel to each otherand that is arranged such that information equal to storage informationstored in the volatile storage circuit is stored into the nonvolatilestorage circuit, the compound storage circuit comprising a determinationcircuit for comparing first storage information stored in the volatilestorage circuit with second storage information that has already beenstored in the nonvolatile storage circuit when the storage informationstored in the volatile storage circuit is written into the nonvolatilestorage circuit, wherein the first storage information is written intothe nonvolatile storage circuit only when the first storage informationis not equal to the second storage information.
 2. The compound storagecircuit according to claim 1, wherein the determination circuitincludes: comparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.
 3. The compound storage circuit according toclaim 1 or 2, wherein the nonvolatile storage circuit includes amagnetic tunnel junction element as storage means.
 4. The compoundstorage circuit according to claim 1, wherein: when electric powersupplied to the volatile storage circuit is reduced, the storageinformation stored in the volatile storage circuit is written into thenonvolatile storage circuit; and when power supply resumes after theelectric power supplied is reduced, the storage information stored inthe nonvolatile storage circuit is returned into the volatile storagecircuit.
 5. The compound storage circuit according to claim 4, whereineach of the volatile storage circuit and the nonvolatile storage circuitincludes power source supply means that operates when the electric powersupplied is reduced.
 6. The compound storage circuit according to claim4 or 5, wherein the nonvolatile storage circuit includes a magnetictunnel junction element as storage means.
 7. The compound storagecircuit according to claim 4 or 5, wherein the determination circuitincludes: comparison determination means for comparing the first storageinformation with the second storage information; and writing means forwriting the first storage information into the nonvolatile storagecircuit only when the first storage information is not equal to thesecond storage information.
 8. The compound storage circuit according toclaim 7, wherein the nonvolatile storage circuit includes a magnetictunnel junction element as storage means.
 9. A semiconductor deviceincluding a compound storage circuit that includes a volatile storagecircuit and a nonvolatile storage circuit connected in parallel to eachother and that is arranged such that information equal to storageinformation stored in the volatile storage circuit is stored into thenonvolatile storage circuit, the semiconductor device comprising adetermination circuit for comparing first storage information stored inthe volatile storage circuit with second storage information that hasalready been stored in the nonvolatile storage circuit when the storageinformation stored in the volatile storage circuit is written into thenonvolatile storage circuit, wherein the first storage information iswritten into the nonvolatile storage circuit only when the first storageinformation is not equal to the second storage information.
 10. Thesemiconductor device according to claim 9, wherein the determinationcircuit includes: comparison determination means for comparing the firststorage information with the second storage information; and writingmeans for writing the first storage information into the nonvolatilestorage circuit only when the first storage information is not equal tothe second storage information.
 11. The semiconductor device accordingto claim 9 or 10, wherein the nonvolatile storage circuit includes amagnetic tunnel junction element as storage means.
 12. The semiconductordevice according to claim 11, wherein: when electric power supplied tothe volatile storage circuit is reduced, the storage information storedin the volatile storage circuit is written into the nonvolatile storagecircuit; and when power supply resumes after the electric power suppliedis reduced, the storage information stored in the nonvolatile storagecircuit is returned into the volatile storage circuit.
 13. Thesemiconductor device according to claim 12, wherein each of the volatilestorage circuit and the nonvolatile storage circuit includes powersource supply means that operates when the electric power supplied isreduced.
 14. The semiconductor device according to claim 12, wherein thenonvolatile storage circuit includes a magnetic tunnel junction elementas storage means.
 15. The semiconductor device according to claim 12,wherein the determination circuit includes: comparison determinationmeans for comparing the first storage information with the secondstorage information; and writing means for writing the first storageinformation into the nonvolatile storage circuit only when the firststorage information is not equal to the second storage information. 16.The semiconductor device according to claim 15, wherein the nonvolatilestorage circuit includes a magnetic tunnel junction element as storagemeans.
 17. The semiconductor device according to claim 13, wherein thenonvolatile storage circuit includes a magnetic tunnel junction elementas storage means.
 18. The semiconductor device according to claim 13,wherein the determination circuit includes: comparison determinationmeans for comparing the first storage information with the secondstorage information; and writing means for writing the first storageinformation into the nonvolatile storage circuit only when the firststorage information is not equal to the second storage information.